A basic role in digestion of carbohydrates in a digestive tract belongs following enzymes: a) Alfa-amylase, hexokinase, lactase b) Maltase, sucrase, enterokinase c) Lactase, alfa-amylase, sucrase d) Sucrase, maltase, glycogen phosphorylase correct option
The correct answer is c) Lactase, alfa-amylase, sucrase.
Reasoning through each option:
Carbohydrate digestion requires enzymes that break down polysaccharides and disaccharides into monosaccharides:
- Alpha-amylase (salivary and pancreatic) - cleaves alpha-1,4-glycosidic bonds in starch and glycogen, producing maltose, dextrins, and oligosaccharides
- Sucrase (brush border enzyme) - hydrolyzes sucrose into glucose + fructose
- Lactase (brush border enzyme) - hydrolyzes lactose into glucose + galactose
- Maltase (brush border enzyme) - hydrolyzes maltose into two glucose molecules
Why the other options are wrong:
| Option | Problem |
|---|---|
| a) Alfa-amylase, hexokinase, lactase | Hexokinase is an intracellular enzyme that phosphorylates glucose (glycolysis), NOT a digestive enzyme |
| b) Maltase, sucrase, enterokinase | Enterokinase (enteropeptidase) activates trypsinogen → trypsin; it is a protease activator, not a carbohydrate-digesting enzyme |
| d) Sucrase, maltase, glycogen phosphorylase | Glycogen phosphorylase breaks down intracellular glycogen in liver/muscle cells; it plays no role in intestinal carbohydrate digestion |
Option c contains three enzymes that all directly participate in intestinal carbohydrate digestion:
- Alpha-amylase - luminal phase (salivary/pancreatic), breaks starch into smaller fragments
- Sucrase - brush border phase, digests dietary sucrose
- Lactase - brush border phase, digests dietary lactose
Which of the following glucose transporter (GLUT) is important in fructose transport in the intestine? a) GLUT1 b) GLUT3 c) GLUT5 d) GLUT7
The correct answer is c) GLUT5.
Explanation:
GLUT5 is the primary transporter responsible for fructose absorption across the apical (luminal) membrane of intestinal enterocytes. It is a facilitative transporter that moves fructose down its concentration gradient from the intestinal lumen into the enterocyte.
Why the others are incorrect:
| Transporter | Primary Location & Function |
|---|---|
| GLUT1 | Ubiquitous (RBCs, brain, placenta) - transports glucose; basal glucose uptake in most cells |
| GLUT3 | Neurons - high-affinity glucose transporter for the brain |
| GLUT5 | Small intestinal enterocytes & kidney - fructose-specific transporter ✓ |
| GLUT7 | Small intestine & colon (intracellular membranes) - transports glucose/fructose intracellularly, not the primary intestinal fructose absorber |
Complete picture of intestinal fructose transport:
Intestinal lumen
↓ GLUT5 (apical membrane)
Enterocyte cytoplasm
↓ GLUT2 (basolateral membrane)
Portal bloodstream
- GLUT5 on the apical side imports fructose into the enterocyte
- GLUT2 on the basolateral side exports fructose (along with glucose and galactose) into the portal circulation
Key fact: GLUT5 has very high specificity for fructose and negligible affinity for glucose, making it the defining intestinal fructose transporter.
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